A Neutrino Experiment With Pion and Muon Decays at rest in J-PARC
Koichiro Nishikawa discussed the proposal to install in the J-PARC facility an experiment to study the issue of sterile neutrinos. He explained that beyond the 3-generation scheme there are several issues: to test the unitarity of the PMNS matrix, and to search for additional neutrinos. One needs to search or refute the LSND excess definitively, and /or determine their origin. J-PARC can contribute in both these areas.
The advantage of decays at rest of pions and muons are multiple. A pulsed beam allows for the separation of positive muon decays from beam decays of neutron and pions, by rejecting events based on the time structure of the beam; the main components are muon antineutrinos and electron neutrinos. Due to the nuclear absorption of negative pions and muons, the electron antineutrino component is reduced by a factor 1000.
The event rate at 1MW for 4000 hours per year of operation is very high: the number for a 1-ton detector at 20 m from the MLF target are 250 per year, and for inverse-beta decay is of 5000 per year. Incidentally, one would get 10^20 positive kaons stopping in the detector, which could enable many different studies.
He discussed the scheme for the target (made of mercury), which is one of the critical items. Also the shielding and the neutron backgrounds are important issues. He then discussed the detector, and the possible five-sigma reach of the setup. He finally mentioned the near-future time-line of the project: in 2014 they should install a 250-liter Liquid-argon detector in the ML facility, to study the kaon stopping rate and nuclear effects, and to identify the source of on-bunch timing activity for a consideration of the full-scale MW facility. In 2015 it could be possible to do an oscillation search.